The capacity of Phytophthora ramorum to colonize the inner bark of 18 native and two exotic tree species from the Iberian Peninsula was tested. Living logs were wound-inoculated in a growth chamber with three isolates belonging to the EU1 and two to the NA1 clonal lineages of P. ramorum. Most of the Quercus species ranked as highly susceptible in experiments carried out in summer, with mean lesion areas over 100 cm2 in Q. pubescens, Q. pyrenaica, Q. faginea and Q. suber and as large as 273 cm2 in Q. canariensis, ca. 40 days after inoculation. Quercus ilex ranked as moderately susceptible to P. ramorum, forming lesions up to 133 cm2 (average 17·2 cm2). Pinus halepensis and P. pinea were highly susceptible, exhibiting long, narrow lesions; but three other pine species, P. pinaster, P. nigra and P. sylvestris, were resistant to slightly susceptible. No significant difference in aggressiveness was found between the isolates of P. ramorum. In addition, there was evidence of genetic variation in susceptibility within host populations, and of significant seasonal variation in host susceptibility in some Quercus species. The results suggest a high risk of some Iberian oaks to P. ramorum, especially in forest ecosystems in southwestern Spain, where relict populations of Q. canariensis grow amongst susceptible understory species such as Rhododendron ponticum and Viburnum tinus. One isolate of P. cinnamomi used as positive control in all the inoculations was also highly aggressive to Iberian oaks and Eucalyptus dalrympleana.